The operating temperature range of such stack assemblies is limited in practice to the operating range of the plastic seal. At low temperatures (-20.degree. C. and below) the seal becomes brittle or its thermal expansion characteristics cease to be adequately matched to those of the metal closure members, and at high temperatures (+90.degree. C. and above) the seal suffers from creep. In either case, the cells are no longer properly sealed, and their performance deteriorates or they cease functioning altogether.
In order to improve the mechanical strength of such stacks, a known practice consists in surrounding them over their entire length by reinforcing members, in inserting the resulting assembly into a tube (leaving two opposite polarity electrical terminals emerging therefrom), and in filling the voids in the tube with an encapsulating compound.
Unfortunately, it has been observed that regardless of the precautions taken during manufacture, there remain sealing problems with the cell or cells in the top stage of the stack.
Preferred embodiments of the present invention remedy this defect and considerably increase the range of temperatures over which the stack of cells may be used.